Università degli Studi Roma Tre, Dipartimento di Matematica e Fisica, Roma, Italia

A combined vertical and oblique radio-soundings data assimilation procedure is proposed for the Regional Assimilative Three-dimensional Ionospheric Model (RATIM). As described in a previous paper [1], RATIM has demonstrated a good degree of adaptability to different ionospheric conditions, when vertical plasma frequency profiles fp(h) over the Italian area are ingested. The fp(h) assimilation procedure consists in minimizing the root-mean-square deviation RMSD between the observed and modeled profiles at the locations where observations are available. This enables the model to adjust the values of some ionospheric parameters previously described on empirical bases, testing a wide set of values for their variations. Hence, such variations are effectively RATIM free parameters, as they are varied until the best fit for the available profiles is obtained. A Maximum Usable Frequencies (MUFs) ingestion technique has been subsequently introduced in RATIM. A simple HF ray-tracing technique has been used to model the ground range D of a particular radio-link, evaluating the skip distance for a signal obliquely transmitted towards a specific ionosphere, when the signal frequency is set equal to the MUF for the radio-link itself. A simplified ionosphere between the transmitter and the receiver is assumed, extending the same parabolic fp(h) to the whole radio-propagation channel. This profile is constrained to some F2 characteristics linked to the RATIM free parameters. A comparison between the real and simulated D values is then performed for each combination of the free parameters tested during the fp(h) ingestion, introducing a further condition to the fp(h) RMSD minimization. Preliminary studies of the application of this method are presented, when the MUF-ingesting version of RATIM has been applied to the Japanese-South Korean region, and the MUF values ingested have been obtained by the Oblique Ionogram Automatic Scaling Algorithm (OIASA) [2, 3]. RATIM adaptability has been tested, comparing the percentages of success of the adjustment procedure when only fp(h) are ingested and applying the MUFs assimilation with different thresholds for the ΔD=|D[real]-D[RATIM]| values to be acceptable. The minimized fp(h) RMSD values have been also compared in such conditions, along with the ΔD values obtained in adapting conditions. The RATIM ability to reject incorrect data has also been tested, when fp(h) and MUF values are validated by an expert operator.

The effect on the ionosphere of solar eclipse of March 20, 2015 on different ionospheric layers is studied. The response of the critical frequencies foF1 and foF2, related to the ionospheric F1 and F2 regions have been investigated during the solar eclipse, using the vertical ionospheric soundings from the ionosondes of Rome, Gibilmanna and San Vito dei Normanni. A further study on the occurrence of the Sporadic E layer during the eclipse hours is here presented.

A regional adaptive and assimilative three-dimensional (3D) ionospheric model is proposed. It is able to ingest real-time data from different ionosondes, providing the ionospheric bottomside plasma frequency fp over the Italian area. The model is constructed on the basis of empirical values for a set of ionospheric parameters Pi[base] over the considered region, some of which are assigned a variation Pi. The values for the ionospheric parameters actually observed at a given time at a given site will thus be Pi= Pi[base]+ΔPi. These Pi values are used as input of an electron density N(h) profiler. The latter is derived from the Advanced Ionospheric Profiler (AIP), which is software used by Autoscala as part of the process of automatic inversion of ionogram traces. The 3D model ingests ionosonde data by minimizing the root-mean-square deviation between the observed and modeled values of fp(h) profiles obtained from the associated N(h) values at the points where observations are available. The Pi values are obtained through such a minimization procedure. The 3D model is tested using data collected at the ionospheric stations of Rome (41.8 N, 12.5 E) and Gibilmanna (37.9 N, 14.0 E), and then comparing the results against data from the ionospheric station of San Vito dei Normanni (40.6 N, 18.0 E). The software developed is able to produce maps of the critical frequencies foF2 and foF1, and of fp at a fixed altitude, with transverse and longitudinal cross-sections of the bottomside ionosphere in a color scale. fp(h) and associated simulated ordinary ionogram traces can easily be produced for any geographic location within the Italian region. fp values within the volume in question can be also provided.